When the quote is not exact, but substitutes the original preposition with what the author actually meant.
Imagine the original text is:
Albert Einstein discovered Theory of Relativity, lay foundations for research on nuclear power and pushed physics by strides. He was genuinely a genius.
Now you want to quote the last sentence only. If you write:
He was genuinely a genius.
that's useless and meaningless, because you skipped, who was that genius. If you write:
Albert Einstein was genuinely a genius.
that's paraphrasing the quote without indicating the paraphrase, a bad habit - you don't change someone's words at random when quoting!
"[Albert Einstein] was genuinely a genius."
This clearly indicates what the original author meant, but also indicates the original quote has been modified for the sake of clarity.
In case of "horrible" it may also be similar. Say, you wrote a very negative review of a trip, a restaurant you ate at, attractions, and you end with
"So, my day's experiences could be summed up as abysmal. The night at the hotel was essentially the same."
Now if someone wants to copy just your quote on the hotel while skipping the rest of your review, they will have to write:
"The night at the hotel was essentially [abysmal]."